Background: Hepatitis C virus (HCV) infection is currently a major health threat among injection drug users (IDUs) in this country. To date, few interventions have specifically targeted HCV, although a great many intervention efforts have targeted human immunodeficiency virus (HIV) transmission. Both of these diseases are transmitted by similar injection-related behaviors, specifically sharing needles and other paraphernalia. In this application we propose to test two interventions found to be effective in reducing HIV-related risk behaviors. These include an individually focused street outreach intervention we have been using in Denver since 1987, and a network oriented peer education intervention developed in Baltimore by Carl Latkin, Ph.D. In both approaches, a cohort of IDUs will be recruited and trained to serve as HCV/HIV prevention experts in the community. These experts, referred to as "index" subjects, will be asked to bring to the project two members of their injecting network. The individually focused intervention will focus on changing the index participants' behavior, while the peer intervention will focus on providing index participants with the education and skills necessary to change the norms of their network. The aims of the project are: 1. To compare the effectiveness of a peer network intervention vs. an individually focused intervention in increasing knowledge about HCV and HIV among both index and network members; 2. To compare the two intervention approaches on both index and network members in reducing HCV/HIV-related risks; and 3. To compare the effectiveness of the two interventions in increasing self-efficacy to reduce HCV and HIV risk-related behaviors among index and network members. A total of 750 IDUs will be randomly assigned to one of the two interventions. Half of these will participate in the peer education arm of the study and half will participate in the individually focused intervention. Two hundred and fifty networks will be examined, again with half in each arm. Index subjects will bring in two network members to participate in the study. Index members will then be randomly assigned to one of the two intervention arms. If in the peer education arm, index subjects will undergo the peer education intervention, consisting of five group sessions focusing on peer education over a period of three weeks. A booster session will also be required. Network members in both arms will be assessed for risk behaviors, self-efficacy, and HCV/HIV knowledge at baseline, six months and 12 months. In the individually- focused arm, index subjects will undergo five individual risk reduction sessions over three weeks time. HCV and HIV pre- and post-test counseling will be required of all subjects at baseline, six months and 12 months.